Removal of Microbeads from Wastewater Using ElectrocoagulationWilliam Perren, Arkadiusz Wojtasik, and Qiong Cai*

Department of Chemical & Process Engineering, Faculty of Engineering and Physical Sciences, University of Surrey, Guildford,Surrey GU2 7XH, U.K.

ABSTRACT: The need for better microplastic removal fromwastewater streams is clear, to prevent potential harm themicroplastic may cause to the marine life. This paper aims toinvestigate the efficacy of electrocoagulation (EC), a well-known and established process, in the unexplored context ofmicroplastic removal from wastewater streams. This premisewas investigated using artificial wastewater containing poly-ethylene microbeads of different concentrations. The waste-water was then tested in a 1 L stirred-tank batch reactor. Theeffects of the wastewater characteristics (initial pH, NaClconcentration, and current density) on removal efficiency werestudied. Microbead removal efficiencies in excess of 90% were observed in all experiments, thus suggesting that EC is an effectivemethod of removing microplastic contaminants from wastewater streams. Electrocoagulation was found to be effective withremoval efficiencies in excess of 90%, over pH values ranging from 3 to 10. The optimum removal efficiency of 99.24% was foundat a pH of 7.5. An economic evaluation of the reactor operating costs revealed that the optimum NaCl concentration in thereactor is between 0 and 2 g/L, mainly due to the reduced energy requirements linked to higher water conductivity. In regard tothe current density, the specific mass removal rate (kg/kWh) was the highest for the lowest tested current density of 11 A/m2,indicating that low current density is more energy efficient for microbead removal.

1. INTRODUCTION

Over the last century, the plastics industry has emerged andgrown into a monolith that affects us at every step of our lives.Over the past 50 years, the use of plastics has increased 20-foldworldwide, with a predicted doubling of plastic usage over thenext 20 years.1 Sadly, this growth in plastic usage has alsomanifested itself via an ever increasing presence of plastics inwaste streams, both on land and in water. It has been estimatedthat there were over 150 million tons of plastic waste in marinewaters as of 2016.2 Approximately, 0.1−1.5% of this waste ismade up of microplastics, which are defined as plastic particlesof less than 5 mm diameter.3 Microplastics can be classified aseither primary or secondary. Primary microplastics areintentionally produced to fill very specific roles in manyindustries. The largest use is in personal care and cosmeticproducts (PCCPs), such as facial scrubs, where around 93% ofall microplastics used in PCCPs are polyethylene-derivedbeads.3 These microplastic particles commonly find their wayinto wastewater streams, pass through wastewater treatmentplants (WWTPs) untreated, and finally end up in marinewaters.4 Secondary microplastics are produced when largerplastic particles that currently find themselves in water streamsbreak apart due to a combination of UV degradation,mechanical stresses, and biological processes.5

Because of their small size, microplastics that are present inmarine waters are easily ingested by the local marine flora. Theimpact of the addition of plastic to the diet of marine organismsis not fully understood. However, evidence has shown thatcosmetic-derived microbeads can transfer adsorbed organic

pollutants to aquatic species that ingest them, making cosmeticmicrobeads a serious but entirely preventable source of marinepollution.6,7

A legislative ban of microbeads in cosmetic products indeveloped countries, such as the United States, has proveneffective.8 However, in developing countries, PCCP regulationis severely deficient9 and they lack both the infrastructure andthe capital to construct centralized WWTPs, includingexpensive tertiary treatment capable of removing microbeadsfrom effluent streams.10 There is a clear need for an innovative,cheap, and energy-efficient solution that can be used bothlocally and to supplant existing tertiary treatment.Electrochemical techniques, such as electrocoagulation (EC),

electrodecantation, and electroflotation, offer a cheaper methodof tertiary treatment that does not rely on chemicals ormicroorganisms, such as in chemical coagulation or activatedsludge processes. Instead, EC uses metal electrodes to producecoagulant electrically, making the process simple and robust.11

The benefits of electrochemical processes, including EC, extendto environmental compatibility, low capital costs, energyefficiency, sludge minimization, amenability to automation,and cost effectiveness.12

The process of EC functions by liberating metal ions fromsacrificial electrodes into the water stream via electrolysis. Theanodic and cathodic reactions are given in eqs 1−2 and 3−4,

Received: December 21, 2017Accepted: March 8, 2018Published: March 20, 2018

Article

Cite This: ACS Omega 2018, 3, 3357−3364

© 2018 American Chemical Society 3357 DOI: 10.1021/acsomega.7b02037ACS Omega 2018, 3, 3357−3364

This is an open access article published under a Creative Commons Non-Commercial NoDerivative Works (CC-BY-NC-ND) Attribution License, which permits copying andredistribution of the article, and creation of adaptations, all for non-commercial purposes.

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respectively. These ions then form coagulants in situ. The mostcommonly used coagulants produced by EC are formed byreaction of the metal ions, usually Fe2+ or Al3+, with OH− ionsformed by electrolysis to produce metal hydroxide coagulants.These coagulants destabilize the surface charges of thesuspended solids, breaking up the colloid or emulsion, whichin turn allows them to approach each other close enough forvan der Waals forces to take effect. Meanwhile, the coagulantforms a sludge blanket, which traps the suspended solidparticles. The H2 gas liberated in the electrolysis process thenlifts the resultant sludge to the water surface.13 Presently, EChas been shown to effectively remove dyes,14 heavy metals,15

and clay particles,16 with >80% of the polluting particlesremoved after treatment. Effective removal of some liquidorganics has also been proven.11 However, the possibility ofusing EC for the removal of microplastics, such as polyethylenemicrobeads, has not been explored. In this context, we proposeto utilize a reactor setup that has not been explored by otherresearch groups, in the bid to reduce the operating cost of themicrobead removal process. This is the niche that this projectattempts to explore. To the author’s knowledge, our paperreports, for the first time, the investigation of using EC formicrobeads removal.

→ ++ −nM M en(s) (aq) (1)

→ + ++ −2H O 4H O 4e2 (l) (aq) 2(g) (2)

+ →+ −nM e Mn(aq) (s) (3)

+ → +− −2H O 2e H 2OH2 (l) 2(g) (4)

+ →+ −nM OH M(OH)nn(aq) (s) (5)

The aim of the presented work is to investigate the feasibility ofusing aluminum-based EC to effectively remove plasticmicrobeads present in both domestic wastewater and industrialeffluent. The study focused on investigating three operationalparameters: initial pH, conductivity (NaCl concentration), andcurrent density, to observe their effects on the microbeadremoval efficiency and to optimize both the microbead removaland energy efficiency. Removal and energy efficiency weremeasured in terms of removal efficiency and specific massremoval of the microbeads, respectively. On the basis of thefindings within this paper, possible operation parameters for theimplementation of EC as part of wastewater treatment wereproposed.

2. RESULTS AND DISCUSSIONFor all of the investigations in this study, substantial removal ofmicrobeads was observed with the EC process, with the highestremoval efficiency being 90−100%. Statistical analysis using thet test adapted for non-normal distributions, the Mann−Whitney U test, shows that the microbead concentration in atreated sample at any given time is always significantly lesserthan the microbead concentration in an untreated controlsample (Mann−Whitney U test; U = 190.5, p < 0.0005). The Uvalue shows the average difference in applied statistical rankbetween treated and control samples, whereas p < 0.05 showsthat these results are significant with a >95% confidence level.2.1. Effect of Initial pH. The initial pH value of the water

was found to be an important operational parameter by otherresearch groups17−19 for turbidity removal in industrial estate

and food processing wastewater and drinking water feeds usingthe EC process. In these studies, there was often a characteristicoptimum pH range discovered for different wastewater types.Figure 1 shows the change in the removal efficiency over time

at different pH values. For all of the pH values investigated (pH= 3, 5, 7.5, and 10), the removal efficiency increases with timeand reaches plateau after 40 min of EC operation. Themaximum removal efficiency achieved is found to be 85−100%,showing that the EC process is effective for microbead removalover a wide range of pH values (pH = 3−10). As seen in Figure1, there is no significant variance in removal efficiency acrossthe tested pH range.Nevertheless, after 60 min of treatment, samples at all pH

ranges expressed successful removal with final removalefficiencies of >85%. From this data, the characteristic optimumpH range for this wastewater analogue is found to be pH = 3−10, indicating that EC is suitable for removing microbeads fromwastewater streams with a wide range of pH values. The pHvalue of the wastewater depends on its origin. For example, thedomestic/municipal wastewater usually has a pH range of 6−9.2.20,21 This flexibility of the EC process with regards to pHmeans that it could be used effectively for all commonwastewater effluent-containing microbeads without requiringthe addition of further chemicals to adjust the pH.Figure 2 shows the final microbead removal efficiency at

different initial pH values after 60 min of EC operation time. Aremoval efficiency of above 89% is successfully achieved for allof the samples at pH values ranging from 3 to 10. The finalremoval efficiencies at pH = 3 and 10 is lower than those at pH= 5 and 7.5. The optimum final removal efficiency of 99% wasfound at pH = 7.5. The results indicate that a more neutral pHis expected to give better removal due to the favorableproduction of coagulant at neutral pH, the same phenomenabeing reported by other research groups.18

At pH = 3, it was also observed that the beginning of flocformation was only visible after 5 min, whereas floc formationwas visible almost immediately (<1 min) at all other initial pHvalues investigated. Previous work has found that for an Al/H2O system at 25 °C, Al(OH)3 will be the predominant speciesabove pH = 3.7.13 Below pH = 3.7, the reaction described by eq5 is less favorable and Al3+ is dominant. Figure 3 shows thechange in pH of the wastewater samples with a constant line atpH = 3.7. The sample with initial pH = 3.0 took approximately

Figure 1. Microbead removal efficiency with time at different startingpH values. All other reactor conditions: current density, 15 mA/cm2;sodium chloride as supporting electrolyte 10 g/L; initial microbead(300−355 μm) concentration, 0.1 g/L; electrode spacing, 1 cm; andstirring speed, 60 rpm.

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5 min to cross pH = 3.7, which agrees with the aboveobservation that flocs began visibly forming after 5 min. Thiswas when the pH was high enough for Al3+ to react favorably toform Al(OH)3 precipitate.Although floc formation is clearly affected by pH, the time-

dependent removal efficiency in Figure 1 shows smalldependence on pH values. Visible observations showed thatcomplete coverage of the reactor in a floc blanket withmicrobeads being seen visibly adsorbed on the flocs occurred at15 min. This indicates that floc production may not be asignificant factor when determining removal in the first 15 min.It is speculated that the mechanism of charge neutralization byAl3+ ions may be as effective as the flocculation mechanism byAl(OH)3 within this time frame. Further research is required tounderstand in detail the dominating mechanism for microbeadremoval at different pH values.2.2. Effect of Conductivity. The effect of conductivity of

the wastewater was investigated by adjusting the concentrationof NaCl in the sample. The minimum and maximum saltconcentrations employed were 2 and 8 g/L, respectively,corresponding to measured wastewater conductivities of 7.44and 13.75 mS/cm. Figure 4 shows the average time-dependentremoval efficiency at each NaCl concentration. Over the rangeof 2−8 g/L, NaCl concentration had no significant effect onremoval efficiency at any time. The microbead removalefficiency increases with time and reaches a plateau after 40

min. All samples treated show >90% final removal efficiencyafter EC treatment for 60 min.The results show that the increased presence of Cl− ions has

minor effect on the removal efficiency (especially for NaClconcentration below 8 g/L), whereas previous studies22 showedheavy dependence of the removal efficiency of the targetedpollutants (dyes) on the concentration of Cl− ions. In theprevious study, it was found that the presence of Cl− ions andformed HOCl resulted in side reactions that decomposed thedyes and aided adsorption onto the formed flocs,22 whereas inthe case of microbead removal, at the pH range that wasinvestigated, there was no evidence to suggest that the removalof polyethylene microbeads was affected by the presence of Cl−

and HOCl species. It is postulated that the time taken forHOCl to cause degradation in the microbeads is far longer thanthe 60 min residence time of the electrochemical reactor and sothe effect of NaCl concentration on the removal efficiency isnot significant.The energy consumption, E(t), of the EC cell was calculated.

Throughout the EC process, the voltage fluctuated with time tokeep a constant density as set by the direct current (DC) powersupply. As such, further calculations will be using the time-averaged voltage applied, V, and the constant current, I. FromE(t) and the absolute removal, Mabs,t (eq 13, found in Section4.3), the specific mass removal per unit energy (g/kJ), Xs(t), ofthe cell was calculated, using eqs 6 and 7.

=E t VIt( ) (6)

=X tM

E t( )

( )t

sabs,

(7)

Figure 5 shows the calculated energy-specific removal over timefor each NaCl concentration studied. It can be seen that specificremoval is improved as NaCl concentration is increased. Thesample containing 8 g/L of NaCl consumed 62% less electricalenergy than that of the sample containing 2 g/L of NaCl. Thisrevealed the possibility of optimizing the energy consumption(which is directly related to the operation cost) based on theaddition of NaCl. According to Ozyonar and Karagozoglu,23

the operating cost per cubic meter of water treated by an ECreactor can be described using eq 8 (adapted to include the costof NaCl). Note that this is simplified and does not consider thecosts of waste sludge removal, cleaning, and maintenance that afull-scale reactor would incur

Figure 2. Final microbead removal efficiency after 60 min ofelectrocoagulation at different initial pH values. Reactor conditionssame as those given in Figure 1.

Figure 3. pH change against electrolysis time at different starting pHvalues (light blue filled circles, pH = 3.0; green filled squares, pH = 5.0;dark blue filled triangles, pH = 7.5; and black filled inverted triangles,pH = 10.0). Reactor conditions are the same as in Figure 2. The redline shows constant pH = 3.7 above which Al(OH)3 is the dominantmetal species.

Figure 4. Microbead removal efficiency against time at different NaClconcentrations. All other reactor conditions: current density, 15 mA/cm2; initial pH = 7.5; initial microbead (300−355 μm) concentration0.1 g/L; electrode spacing, 1 cm; and stirring speed, 60 rpm.

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= + +X Y Zoperating cost energy salt electrodecons cons cons

(8)

The salinity of wastewater has been found to have no significanteffect on electrodissolution rate.13 This optimization issimplified to only consider the effect of NaCl concentrationon operating cost, therefore revealing the optimum NaClconcentration for any other combination of operatingconditions. Because electrode consumption and its associatedcosts do not depend on NaCl concentration, eq 8 can besimplified to

= +X Yoperating cost energy saltcons cons (9)

where energycons and saltcons are the consumptions of electricityand NaCl per m3 of water treated, respectively. X and Y are theunit costs of electricity (p/kWh) and NaCl (p/kg), respectively.Figure 6 shows the estimated operating costs as a function of

salt concentration for an EC cell operating for 60 min, at a

current density of 15 mA/cm2, an electrode spacing of 1.5 cm,and stirring speed of 60 rpm. As the NaCl concentration inwastewater increases, operating cost increases. This is a result ofthe increase in the amount (therefore, cost) of NaCl salt addedto achieve the corresponding NaCl concentration. Figure 6shows that for NaCl concentration higher than 3 g/L, thelargest factor affecting cost is the salt consumption per m3 ofwater treated.Figure 6 also shows that the cost of electricity is the

dominant factor in determining the operation cost, when the

NaCl added is below 3 g/L. Of all of the concentration valuestested, the minimum operating cost (for factors related to saltconcentration) for microbead removal was found to be at 2 kg/m3 NaCl.

2.3. Effect of Current Density. Current density is a keyparameter in the application of EC as it is an operatingparameter that can be directly controlled using the DC powersupply.13,25 Figure 7 shows the average microbead removal

rates for the different tested current densities. Between 11 and23 A/m2, there was no distinct difference in removal ratesbetween the current density values tested at any specific time.According to Faraday’s law of electrolysis (eq 10), it is

expected that increasing current density of the cell will result inan increase in metal ions liberated from the electrodes. Thismeans that high amount of metal ions and therefore flocculantsis expected to be present at high current density.

=mItMFz (10)

Figure 7 shows that removal efficiency of microbeads by EC isnot significantly affected by the change in current density. Thisindicates that the amount of metal ions does not cause obviousremoval of microbeads for the range of current densitiesinvestigated. It is clear that flocculation and settling appear tobe the dominant mechanism of microbead removal when anexcess of coagulant has been produced. The time at whichcoagulants are in excess and flocculation mechanisms dominateappears to be between 0 and 30 min as current density appearsto have a more important effect on microbead removal over thistime frame. This is shown by the decrease in gradient of theremoval efficiency lines after 30 min in Figure 7. This impliesthat operating the reactor for more than 30 min would createexcess coagulant with little effect on removal efficiencycompared to allowing it to settle after 30 min. Meanwhile,longer operation will result in more reactor waste and greaterelectrode and energy consumption.The most energy-efficient current density for microbead

removal in this particular EC cell was found by comparing thespecific mass removed per kJ of energy spent against currentdensity. Figure 8 shows that at higher current densities thespecific removal of the microbeads is reduced, largely due to theincrease in energy consumption at higher current densities butyielding no significant increase in removal efficiency.From Figure 8, the lowest current density tested, 11 A/m2,

offers the highest energy-efficient microbeads removal over the1 h operational time. It is speculated that the most efficient

Figure 5. Specific microbead removal (g/kJ) against time at differentNaCl concentrations. The reactor conditions are the same as Figure 6.

Figure 6. EC reactor operating costs per m3 of water treated againstdifferent NaCl concentrations. Operating cost calculations done usingeqs 9−12. Unit cost of electricity based on cost of electricity (9.83 p/kWh) for industrial consumers.24 Unit cost of NaCl = 0.798 p/kg.

Figure 7. Average fractional removal for varied current densities.

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current density may be lower than the range currently tested.On the basis of these results, we can conclude that currentdensity does not affect the removal efficiency of the EC cell butoperating at a lower current density will improve the energyefficiency of the cell. However, changing current density willalso determine at what time the reactor will start producing anexcess of coagulant, which will not further improve removalefficiency and will incur excess of reactor waste and increasedelectrode consumption. The time at which the reactor beginsproducing in excess will be specific to the reactor conditions,specifically current density, and can be used to determine thebest operating time for the reactor in question.2.4. Comparison with Previous Literature. Table 1

summarizes the results from recent studies involving electro-coagulation of various pollutants. It is noted that there is noother study in the literature using EC for the removal ofmicrobeads. Therefore, direct comparison of our results withthe literature is not possible. As shown in Table 1, the pollutantremoval rate (typically above 90%) and optimum pH (∼7)from other work agree very well with our work. The apparatuswe designed in this work has, however, managed to achieve aconsiderable reduction in operating cost compared to that bymost other research groups. This is mainly attributed to thetype of pollutants and the fact that microbeads undergo rapidcharge neutralization during the first 15 min, which is able toquickly remove the 50−80% of the initial load of microbeadswithin this time frame. In the remaining operating period,flocculation mechanisms dominate and act to polish thewastewater and remove the remaining microbeads trapped incolloidal suspension. The combined mechanisms allow forsubstantial removal compared with previous research while also

requiring less power to operate. It is recommended that, byexploiting the combination of these effects, low-cost treatmentof microbead-laden streams is feasible. Further work wouldinvolve optimization of the operation time so that power andelectrode consumption could be further minimized.

3. CONCLUSIONSIn this study, the removal of spherical microbeads fromsimulated wastewater by electrocoagulation using aluminumelectrodes was investigated. The effects of pH, conductivity,and current density were studied in an electrochemical batchreactor in bipolar, parallel configuration. The results showedthat EC is an effective method of removing microbeads fromsimulated domestic wastewater. Observations showed thatmicrobeads underwent both flocculation and charge neutraliza-tion simultaneously. By configuring the reactor to exploit bothmechanisms, the reactor operating costs are reduced. For therange of parameters investigated, the optimum reactorconditions were found to be pH = 7.5, NaCl concentration =2 g/L, and current density = 11 A/m2. Future investigations arerecommended to examine the effects of further reducing NaClconcentration and current density on the EC operationefficiency and cost. The most viable option for a large-scaleindustrial EC cell for removing microbeads seems to be a two-stage, continuous EC reactor/settler unit. Further researchshould look at possible reactor designs and configurations tooptimize the process.

4. MATERIALS AND METHODSElectrocoagulation was conducted in a bench-scale stirred-tankbatch reactor (details are given in Section 4.1). Wastewateranalogue (1 L, details are given in Section 4.2) was added andelectrodes placed in parallel along the reactor. Figure 9 showsthe schematic of the reactor setup, connected to a DC powersupply, which controls the voltage and current density of theEC operation. The reactor was run for 60 min for eachexperiment and then turned off, after which the contents wereallowed to settle for 16 h.Three investigations were conducted separately, each with

different independent variables. The values of the controlledvariables along with the range of variables studied for eachinvestigation are given in Table 2.

4.1. Electrochemical Reactor. A 1 L rectangular tank (233mm × 130 mm × 100 mm) was used as the reactor containingwastewater analogue. Seven metal electrodes each of 90 mm ×30 mm × 1 mm were cut from the same 1 mm thick aluminumsheet. The electrodes were placed in the reactor vessels in

Figure 8. Specific microbead mass removal for different currentdensities.

Table 1. Table Comparing Various EC Investigations Found in the Recent Literature

reference type of pollutantselectrodematerial electrode configuration

pollutantremoval rate

CODremoval

TSSremoval

optimuminitial pH

operating cost(per m3)

thiswork

PE microbeads Al bipolar 99% n/a n/a 7.5 £0.05

26 dye (RR198) Al monopolar-parallel 98.6% 84% 98.6% not tested $0.2623 domestic wastewater Al monopolar-parallel 98% 72% 98% 7.8 $0.8627 fluoride Al bipolar >85% n/a n/a 7 3.43 kWh28 iron Al perforated plate flow

column98.5% n/a n/a 6 $0.22

29 bleaching effluent Al monopolar-series n/a 90% 94% 7 $1.5630 strontium stainless steel monopolar-series 93% n/a n/a 5 ≈$2.3031 paint manufacturing

wastewaterAl monopolar-parallel n/a 94% 89% 6.95 €0.13

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parallel configuration suspended by a nonconductive plastic rodof 10 mm diameter. The two outermost electrodes were fixedto copper wires by pop-rivet connectors leading to a FarnellLtd. triple output (4−6, 5−17 V) DC power supply. Only theoutermost electrodes, one acting as the primary anode and theother as the primary cathode, were connected directly to thepower supply giving a parallel, bipolar configuration. The fiveunconnected electrodes in between the anode and the cathodewould then act as bipolar sacrificial electrodes. This resulted ina parallel, bipolar electrode setup. Bipolar configuration waschosen because it appeared to offer better removal rates inother EC investigations.32 The use of unconnected sacrificialelectrodes was preferable as it reduced the number ofinterelectrode connections making the reactor easier to set upand clean in between experiments, while still providing a sourceof aluminum ions (albeit not as strong a source as a connectedelectrode). The connections between the powered electrodeswere reversed after each experiment to prevent excessive,preferential, oxide (passive) layers forming on one electrode.The mixing within the reactor was achieved by a FisherScientific magnetic stirrer set at 60 rpm (placed at the bottomof the reactor) to evenly disperse the formed flocs.

This EC reactor as described above is also suitable for theremoval of other substances from wastewater. In someunpublished work, we have used this system to improve thequality of the opaque water from a local lake. The EC processusing this system successfully removed the organic matters andparticles contained in the water, turning the lake water fromopaque to transparent.

4.2. Wastewater Analogue. To gain a better under-standing of the effects of water properties, a wastewateranalogue was produced to emulate the conditions of domesticwastewater while allowing for complete control over thevariables. In this study, four variables affecting the wastewaterproperties are investigated, including pH, conductivity of water,and the concentration and particle size of microbeads.The wastewater analogue was made up using industrial fresh

water with an average conductivity of 447 μS/cm and a pH of7.5. HCl (1 M) and NaOH (1 M) solutions were used tocontrol the pH without allowing a buffer to form in the initialsample. NaCl crystals (99.5% w/w, Fisher Chemicals) wereadded to 1 L of the water to adjust conductivity as required.Fluorescent green, spherical microbeads of 300−355 μm

(0.997 g/cm3) were used. According to the supplier (CosphericLLC), >90% of the microbeads, which are used in industry andend up in wastewater streams, were expected to have similarsize ranges; also >90% of the microbeads are spherical. Theconcentration of microbeads was controlled to be 0.1 g/L forthis investigation.The polyethylene beads were hydrophobic at the time of

manufacture. To ensure that the microbeads are fully dispersedin water, 2 g of cussons morning fresh washing up liquid, actingas a source of surfactants, was added to every 1 L of wastewatersample. The contained surfactants emulate an average domesticwastewater surfactant concentration of 300 mg/L33 and aid themicrobeads to evenly form a suspension as they would in real

Figure 9. Schematic of bench-scale reactor setup used in the investigation.

Table 2. Parameter Values Studied for the Three SeparateEC Investigations

investigation

controlvariablevalues pH NaCl concn current density

electrodespacing

pH 3, 5, 7.5, 10 7.5 7.5 7.5NaCl concn(g/L)

10 2, 4, 6, 8, 10 10 10

currentdensity(A/m2)

15 15 11, 15, 19, 23 15

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wastewater. Figure 10 shows the image of the as-madewastewater analogue with microbeads well dispersed.

4.3. Variable Measurements. The EC reactor was run for60 min with water property measurements taken at every 5 mininterval from 0 to 20 min, then followed by further sampling at30, 45, and 60 min. Temperature and conductivity werecontinually monitored using a Mettler Toledo Five Easy Plusconductivity meter (calibrated using 0.1 g/L of KCl calibratingsolution). pH was measured at each interval using ±0.5accuracy pH indicator sticks (Fisher Scientific). Current andvoltage readings were taken from the power supply readout ateach sample time.As EC progressed, the wastewater analogue became visibly

more turbid due to the formation of a polymeric floc structurethought to be Al(OH)3. Mixing at 60 rpm dispersed these flocsrelatively easily throughout the vessel, and some microbeadswere visibly seen attached to the flocs. After 16 h settlementfollowed the EC process, the contents of the reactor settled.The formed floc blanket sank to the bottom of the reactor andtook with it most of the contained microbeads. The remainingliquid bulk was visibly more clear and free of microbeadscompared to that of the original samples.The removal of microbeads was tracked by taking samples

from the reactor during the EC operation. A representative 20mL of sample was extracted from the bulk of the liquid by a 20mL plastic syringe. The samples then underwent gravityfiltration through Grade 1 Whatman filter papers and thenwere dried at 20 °C for 16 h. The number of microbeads, Nt,for each dried sample was counted. The estimated bulkconcentration for the particle diameter, dp, at the sample timewas then calculated by

ρπ=C N d10012t tmb, p

3(11)

For 300−355 μm microbeads employed in this investigation, anaverage dp = 327.5 μm was used in calculation, where ρ is thedensity of the polyethylene microbeads and Cmb is themicrobead concentration in the bulk of the liquid (g/L).

Absolute mass removal, Mabs,t, at sample time, t, was calculatedby

= −M C Ct tabs, m,0 m, (12)

And the removal efficiency, Mr,t, is calculated by

=−

=MC C

C

M

Ctt t

r,m,0 m,

m,0

abs,

m,0 (13)

4.4. Reactor Maintenance. The reactor vessel was rinsedafter each experiment with distilled water, and the electrodeswere cleaned by acid bath in 1 M hydrochloric acid for 30 min.After being rinsed clean with deionized water, the edges weredeburred using a file. This was done to remove much of theoxide layer formed during experiments, which would preventelectrode passivation from affecting the removal efficiency.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: q.cai@surrey.ac.uk. Tel: 0044 (0)1483 686561.ORCIDQiong Cai: 0000-0002-1677-0515NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe would like to extend special thanks to the Department ofChemical and Process Engineering at the University of Surreyfor providing the funding for this project. The same applies toChris Burt and Ben Gibbons, the lab technicians at theaforementioned department. Their help and advice in regardsto the reactor design as well as reagent procurement has beeninvaluable.

■ REFERENCES(1) World Economic Forum. The New Plastics Economy: Rethinkingthe Future of Plastics; WEC: Davos-Klosters, 2016.(2) EPA. Plastic Microbeads in Products and the Environment; EPA:Sydney, 2016.(3) Gouin, T.; Avalos, J.; Brunning, I.; Brzuska, K.; de Graaf, J.;Kaumanns, J.; Koning, T.; Meyberg, M.; Rettinger, K.; Schlatter, H.;Thomas, J.; van Welie, R.; Wolf, T. Use of Micro-Plastic Beads inCosmetic Products in Europe and Their Estimated Emissions to theNorth Sea Environment. SOFW 2015, 1−33.(4) Elias, S. A. Plastics in the Ocean. Encycl. Anthropocene 2018, 1,133−149.(5) Imhof, H. K.; Laforsch, C.; Wiesheu, A. C.; Schmid, J.; Anger, P.M.; Niessner, R.; Ivlevab, N. P. Pigments and plastic in limneticecosystems: A qualitative and quantitative study on microparticles ofdifferent size classes. Water Res. 2016, 98, 64−74.(6) Farrell, P.; Nelson, K. Trophic level transfer of microplastic:Mytilus edulis (L.) to Carcinus maenas (L.). Environ. Pollut. 2013, 177,1−3.(7) Napper, I. E.; Bakir, A.; Rowland, S. J.; Thompson, R. C.Characterisation, quantity and sorptive properties of microplasticsextracted from cosmetics. Mar. Pollut. Bull. 2015, 178−185.(8) Wardrop, D.; Bott, C.; Criddle, C.; Hale, R.; McDevitt, J.; Morse,M.; Rochman, C. Technical Review of Microbeads/Microplastics in theChesapeake Bay; STAC: Edgewater, MD, 2016.(9) Leslie, H. Review of Microplastics in Cosmetics; IVM Institute forEnvironmental Studies, 2014; Vol. 476, pp 1−33.(10) Lohr, A.; Savelli, H.; Beunen, R.; Kalz, M.; Ragas, A.; VanBelleghem, F. Solutions for global marine litter pollution. Curr. Opin.Environ. Sustainability 2017, 28, 90−99.

Figure 10. Image of a beaker containing wastewater analogue beforethe EC process.

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(11) Garcia-Segura, S.; Eiband, M. M. S. G.; Viera de Melo, J.;Martinez-Huitle, C. A. Electrocoagulation and advanced electro-coagulation processes: A general review about the fundamentals,emerging applications and its association with other technologies. J.Electroanal. Chem. 2017, 801, 267−299.(12) Zeboudji, B.; Drouiche, N.; Lounici, H.; Mameri, N.; Ghaffour,N. The Influence of Parameters Affecting Boron Removal byElectrocoagulation Process. Sep. Sci. Technol. 2013, 48, 1280−1288.(13) Moussa, D. T.; Al-Marri, M. J.; El-Naas, M. H.; Nasser, M. Acomprehensive review of electrocoagulation for water treatment:potentials and challenges. J. Environ. Manage. 2017, 186, 24−41.(14) Khemila, B.; Merzouk, B.; Chouder, A.; Zidelkhir, R.; Leclerc, J.;Lapicque, F. Removal of a textile dye using photovoltaic electro-coagulation. Sustainable Chem. Pharm. 2018, 7, 27−35.(15) Akbal, F.; Camci, S. Copper, chromium and nickel removal frommetal plating wastewater by electrocoagulation. Desalination 2011,269, 214−222.(16) Adapureddy, S. M.; Goel, S. In Optimizing Electrocoagulation ofDrinking Water for Turbidity Removal in a Batch Reactor, InternationalConference on Environmental Science and Technology; IACSITPress: Singapore, 2012.(17) Janpoor, F.; Torabian, A.; Khatibikamal, V. Treatment oflaundry waste-water by electrocoagulation. J. Chem. Technol. Biotechnol.2011, 86, 1113−1120.(18) Yavuz, Y.; Ogutveren, U. B. Treatment of industrial estatewastewater by the application of electrocoagulation process using ironelectrodes. J. Environ. Manage. 2018, 207, 151−158.(19) Banerji, T.; Chaudhari, S. Arsenic removal from drinking waterby electrocoagulation using iron electrodes- an understanding of theprocess parameters. J. Environ. Chem. Eng. 2016, 4, 3990−4000.(20) Ahsan, M.; Riaz, A.; Jaskani, M. J.; Hameed, M. Physiologicaland Anatomical Response of Fragrant Rosa Species with Treated andUntreated Wastewater. Int. J. Agric. Biol. 2017, 19, 13.(21) Idris-Nda, A.; Aliyu, H. K.; Dalil, M. The challenges of domesticwastewater management in Nigeria: A case study of Minna, centralNigeria. Int. J. Dev. Sustainability 2013, 2, 1169−1182.(22) Pirkarami, A.; Ebrahim Olya, M. Removal of dye from industrialwastewater with an emphasis on improving economic efficiency anddegradation mechanism. J. Saudi Chem. Soc. 2017, 21, S179−S186.(23) Ozyonar, F.; Karagozoglu, B. Operating Cost Analysis andTreatment of Domestic Wastewater by Electrocoagulation UsingAluminum Electrodes. Pol. J. Environ. Stud. 2011, 20, 173−179.(24) Annut, A. Prices of fuels purchased by non-domestic consumersin the UK [data set], 2013. https://www.gov.uk/government/statistical-data-sets/gas-and-electricity-prices-in-the-non-domestic-sector (accessed May 08, 2017).(25) Nandi, B. K.; Patel, S. Effects of operational parameters on theremoval of brilliant green dye from aqueous solutions by electro-coagulation. Arabian J. Chem. 2017, 10, S2961−S2968.(26) Dalvand, A.; Gholami, M.; Joneidi, A.; Mahmoodi, N. M. DyeRemoval, Energy Consumption and Operating Cost of Electro-coagulation of Textile Wastewater as a Clean Process. Clean: Soil, Air,Water 2011, 39, 665−672.(27) Palahouane, B.; Drouiche, N.; Aoudj, S.; Bensadok, K. Cost-effective electrocoagulation process for the remediation of fluoridefrom pretreated photovoltaic wastewater. J. Ind. Eng. Chem. 2015, 22,127−131.(28) Hashim, K. S.; Shaw, A.; Al Khaddar, R.; Pedrola, M. O.; Phipps,D. Iron removal, energy consumption and operating cost ofelectrocoagulation of drinking water using a new flow column reactor.J. Environ. Manage. 2017, 189, 98−108.(29) Sridhar, R.; Sivakumar, V.; Immanuel, V. P.; Maran, J. P.Treatment of pulp and paper industry bleaching effluent byelectrocoagulant process. J. Hazard. Mater. 2011, 186, 1495−1502.(30) Murthy, Z. V. P.; Parmar, S. Removal of strontium byelectrocoagulation using stainless steel and aluminum electrodes.Desalination 2011, 282, 63−67.(31) Akyol, A. Treatment of paint manufacturing wastewater byelectrocoagulation. Desalination 2012, 285, 91−99.

(32) Palahouane, B.; Drouiche, N.; Aoudj, S.; Bensadok, K. Cost-effective electrocoagulation process for the remediation of fluoridefrom pretreated photovoltaic wastewater. J. Ind. Eng. Chem. 2015, 22,127−131.(33) Tomczak-Wandzel, R.; Dereszewska, A.; Cytawa, S.; Swarzewo,M. K. A. The effect of surfactants on activated sludge process. JointPolish-Swedish Reports, 2009; pp 1−8.

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